The present invention relates to an apparatus for determining physical and/or chemical and/or biological properties of a medium.
The medium whose physical, chemical and/or biological properties are to be determined by a generic method is preferably a liquid or a gas or a soft material, in particular a highly viscous, dough-like or pasty medium. The acoustic waves used to determine the properties are ultrasonic waves, for example, which are produced by an appropriate transmitter by way of a transmission signal.
In a known method for determining physical, chemical and/or biological properties of the medium, at least two acoustic waves, for example, are produced by a transmission signal, said acoustic waves propagating at least partly through the medium along identical or different propagation directions before they are each received at a receiver lying in the respective propagation direction. By way of example, in a flowing medium, acoustic waves are produced along a first propagation direction in the flow direction of the medium on the one hand and along a second propagation direction counter to the flow direction of the medium on the other hand. Then, a time-of-flight difference can be established from the reception signals generated at the respective receivers and this can be used to deduce the (mean) flow speed of the medium, for example. If, as an alternative or in addition thereto, times of flight and/or amplitudes and/or frequency changes of an acoustic wave from a transmitter to a receiver are still established with the aid of the reception signals, it is possible to draw further conclusions about physical, chemical and/or biological properties of the medium, such as the density, temperature or composition thereof, for example.
WO 2008/034878 A2 has disclosed an apparatus in which acoustic surface waves are produced, said surface waves coupling volume acoustic waves into the respective medium in a waveguide. By repeatedly output coupling surface waves at the sites at which the volume acoustic wave strikes a wall surrounding the medium, acoustic surface waves are received at a receiver in turn, the times of flight and time-of-flight differences of said surface waves being characteristic for the medium and the physical, chemical and/or biological properties thereof.
Consequently, in an apparatus described in WO 2008/034878 A2 and in the method implemented therewith, the processing of the reception signals, produced at the respective receivers, for a received acoustic wave—an acoustic surface wave in this case—plays a decisive role. Thus, establishing a time-of-flight difference or an absolute time of flight from the reception signals produced at the receivers is by no means trivial and, in certain circumstances, connected to significant computational outlay. Very different methods for signal processing are used, depending on the information to be extracted from the reception signals. By way of example, the use of modulated transmission signals to be able to deduce the properties of the medium in a more reliable fashion on the basis of the obtained reception signals is known.
Apart from that, there are ultrasonic methods for measuring the sound speeds in fluids, whether by a direct beam method or else by means of leaky Lamb wave based systems. The advantage of systems based on leaky Lamb waves lies in the great robustness in relation to dispersing particles or bubbles since, in comparison with the direct beam method, there is a significantly higher chance of components of the emitted sound packet also reaching the receiver. In addition to the sound speed, the temperature and, in particular, the substance density, too, are important measurement variables if a substance composition should be examined. There likewise are a number of different known measurement appliances for measuring the substance density. Within certain limits, the substance density likewise can be determined by Lamb wave sensors. However, a disadvantage of the available Lamb wave sensors lies in their comparatively large size, as result of which the access to many fields of applications is difficult. By way of example, long sensor embodiments, which can make the installation process more difficult, are required in pipes with a large nominal width on account of the beam paths. Moreover, an arrangement with opposing plates can make cleaning more difficult.